The present invention relates to encoders and decoders, and more particularly to DC-level constrained coding and decoding.
In magnetic recording media, such as hard disks for computers, a transducer is positioned adjacent to a magnetic recording media. In longitudinal recording, the transducer records data by magnetizing the magnetic recording media in a longitudinal direction. In other words, the transducer magnetizes parallel to the direction of relative movement between the magnetic medium and the transducer. In perpendicular recording, the transducer records data by magnetizing the magnetic recording media in a perpendicular direction.
Computers manipulate information using binary symbols that include the alphabet {1,0}. When writing to the magnetic media, computers magnetize the magnetic media in opposite magnetic directions. To more accurately reflect the bipolar physical magnetization, the information signals are represented by the alphabet {1,xe2x88x921}.
Each bit storage location on a disk drive is a magnetic domain that includes a number of grains, which are crystals of magnetic material. Perpendicular recording allows a smaller grain size than longitudinal recording before encountering a superparamagnetic effect. Perpendicular recording allows tracks that are more narrow and well defined than those in longitudinal recording. In other words, perpendicular recording allows significantly higher areal densities.
The write signals in longitudinal and perpendicular recording are also different. Referring now to FIG. 1, a write signal 10 in longitudinal recording is normally at a DC null 12. The write signal 10 transitions from the DC null 12 to +a or xe2x88x92a during a transition 14 and returns to the DC null 12 until a subsequent transition occurs. In perpendicular recording, however, a write signal 16 transitions between +a or xe2x88x92a. In other words, the write signal 16 does not return to the DC null 12.
In perpendicular recording, a preamplifier and input AC coupling introduces DC distortion. The distortion is monitored and compensation is provided. As the number of consecutive a""s or xe2x88x92a""s increases, a DC offset of the perpendicular recorder tends to drift. In addition, the number of alternating transitions between adjacent symbols should be limited to the extent possible.
A method and apparatus according to the present invention constrains a DC level of an input word. The input word is divided into a plurality of components that include n symbols. The n symbols are summed for each component. An absolute value of the sum is compared to a threshold. The component is encoded into a substitute component if the absolute value of the sum for the component exceeds the threshold. The components having the absolute value that does not exceed the threshold are combined with at least one substitute component into an output word.
In other features of the invention, the substitute component includes less than n symbols. The input word includes 32 symbols and the output word includes at least 33 symbols. The components include 8 symbols and the substitute component includes 5 symbols.
In still other features, an output word template is selected based on a number of substitute components and based upon a position that the substitute components originally occupied in the input word. The substitute components are inserted in the output word based on the output word template. The components that have the absolute value that does not exceed the threshold are inserted in the output word based on the output word template. Address and indicator symbols are inserted in the output word.
In other features, a parity symbol is added to the output word to make a product of symbols of the output word positive or negative.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.